1. Field of the Invention
The present invention relates to a process for the preparation of N-substituted maleimides.
2. Prior Art of the Invention
Various processes have traditionally been employed for preparing an N-substituted maleimide. In a generally known preparation process, maleic anhydride is reacted with a primary amine and the resulting N-substituted maleamic acid is imidized by cyclodehydration.
For example, N-phenyl maleimide can be prepared by an imidizing reaction through intramolecular cyclodehydration as follows: ##STR1##
These processes can be roughly divided into those using a dehydrating agent and those using a catalyst.
The method which uses a dehydrating agent carries out the reaction by employing an equimolar amount or more of a dehydrating agent such as acetic anhydride as disclosed, for example, in U.S. Pat. No. 244,453 or Organic Syntheses, 41, 93. The method is excellent with respect to mild reaction conditions and relatively high yield of the reaction product. However, the method requires a large amount of expensive dehydrating agent and a complex treatment for separating the product after finishing the reaction. Consequently, the method leads to high production cost of the maleimide and is unsuitable for commercial production.
The dehydrating imidization method which uses a catalyst does not use a large amount of expensive auxiliary materials and is essentially an excellent economical method. The dehydrating reaction can occur merely by heating the maleamic acid in the presence of a solvent and the desired product maleimide is formed to some extent when heating is continued to remove the water of reaction from the reaction system. However, the rate of formation is too slow and impractical. Moreover, various unfavorable side reactions also occur and lead to lower selectivity. Accordingly, a catalyst is required for accelerating the dehydrating reaction and improving conversion and selectivity.
As a method using a catalyst, Japanese Patent Publication 51-40078 (1976) discloses a method for carrying out the imidization by dehydrating the maleamic acid in the presence of an acid catalyst and performing intramolecular cyclization by azeotropic distillation of the resulting water. The acid catalysts used are inorganic acids, such as sulfuric acid and phosphoric acid, and strong organic acids, such as p-toluenesulfonic acid having pka of 3 or less. Japanese Patent Publication 55-46394 (1980) describes a method using a solvent system obtained by incorporation of an aprotic polar solvent with an azeotropic solvent in order to enhance the solubility of the maleamic acid. There are also known methods, such as Japanese Patent Laid-Open No. 60-11465 (1985), which use strong acid type ion exchange resin as an acid catalyst and a method for directly obtaining the maleimide by reacting maleic anhydride with primary amine in the presence of the strong acid catalyst mentioned above. Other methods employ the acid catalyst in combination with various additives. For example, Japanese Patent Publication 51-40078 (1976) carries out the reaction by the addition of a stabilizer such as a polymerization inhibitor and an alcohol in the presence of the acid catalyst. Japanese Patent Laid-Open Nos. 61-5066 (1986) and 62-63562 (1987) conduct the reaction in the presence of such a strong acid catalyst as sulfuric acid or phosphoric acid by incorporating one or more metal compounds of zinc, chromium, cobalt, nickel, iron, aluminum or palladium in a trace amount of 0.005 to 0.5%, preferably 0.01 to 0.1% by mole per mole of aniline.
Japanese Patent Laid-Open No. 61-60647 (1986) reacts maleic anhydride with primary amine in an organic solvent to obtain a maleamic acid slurry and then intermittently or continuously charges the slurry to a second reaction vessel where the organic solvent is refluxing in the presence or absence of a catalyst. The catalyst is claimed as follows: "The suitable catalyst used is an oxyacid containing phosphorus or sulfur, or an alkaline metal salt or an alkaline earth metal salt thereof. In addition, salts, hydroxides, oxides and halides of the same metals (Ni, Co, Cu, Zn, Sn, Al, B, Sb, Li, Mg, Cr, Ti, V, Mn and Fe) as those of catalysts usually used for esterification, or a montmorillonite catalyst can also be used as a catalyst." In the detailed specification disclosed as exemplary catalysts are many kinds of strong acids, and sodium salts and magunesium salts thereof. However, no description is given of a specific metal compound as the catalyst, but the above claim is only described again about metal compounds. Some kinds of acid catalysts are used in the examples, but no example is disclosed which uses such metal compounds. Since the above patent application has merely claimed many metal compounds, in addition to the acid catalyst, as a dehydrating imidization catalyst without any technical disclosure and, moreover, by wrong understanding as mentioned hereafter, it is not recognized at all as the so-called prior art in the aspect of metal-catalyzed reactions.
The common essential element for these known methods of catalytically dehydrating imidization is the use of an acid catalyst having strong Broensted acidity, for example, inorganic protic acids, such as sulfuric acid, phosphoric acid, hydrobromic acid and fluorosulfonic acid, and organic protic acids, such as chloroacetic acid, fluoroacetic acid, trifluoromethanesulfonic acid, p-toluenesulfonic acid and sulfonic acid type ion exchange resins.
The process for preparing the N-substituted maleimide by catalytically conducting dehydrating imidization of the N-substituted maleamic acid is a direct reaction which does not consume auxiliary materials and hence is a substantially economical process. Accordingly, many acid-catalyzed methods are known, as mentioned above. However, none have been technically or economically satisfactory for commercial production of the N-substituted maleimides.